1. Field of the Invention
This invention relates generally to the manufacture of ceramic items and particularly to improvements in the manufacture of thin flat ceramic substrates.
2. Description of the Prior Art
The art of manufacturing thin flat ceramic items has been developed to meet the need for capacitor dielectrics and for substrates in microelectronic circuitry, among other uses. The common method for manufacturing such articles is by the tape casting process. In this process a fluid ceramic slurry, called a slip, is made by mixing together a finely ground ceramic powder, such as aluminum oxide (alumina), with suitable amounts of an organic binder, a volatile solvent, usually a plasticizer, and possibly small amounts of other materials, depending on the product requirements. Typical ingredients are identified, and the mixing and casting process are fully described in U.S. Pat. No. 2,966,719, issued to J. L. Park, Jr. on Jan. 3, 1961 as assignor to American Lava Corporation, and in U.S. Pat. No. 3,698,923, issued to H. W. Stetson, et al. on Oct. 17, 1972 as assignors to Western Electric Company, Inc. The reader is referred to these patents and also to the earlier U.S. Pat. No. 2,585,993, issued to G. N. Howatt on Jan. 22, 1952, for details of the tape casting process and its developmental history as well as for the characteristics of the resulting ceramic product.
Briefly, the process involves discharging the above described ceramic slip, which has a viscosity and consistency approximately the same as heavy cream, from a reservoir onto a supported, moving surface, preferably a plastic tape or film such as cellulose acetate, polytetrafluoroethylene ("Teflon"), or glycol terephthalic acid ester ("Mylar"). The film is usually in the form of an elongated strip several hundred feet long and from 1/2 to 2 feet wide, wound on a storage reel mounted next to the reservoir.
The tape is fed from the storage reel under the reservoir to a takeup reel, and a suitable drive mechanism moves the tape in a substantially horizontal path from the storage reel to the takeup reel. The cast slip is distributed evenly on the moving tape by an inverted dam forming the outlet of the reservoir or by a doctor blade in order to form a layer of uniform and controlled thickness.
As the layer of cast slip is conveyed on the plastic tape from the reservoir, the volatile solvents evaporate, the process of driving off the solvents being accelerated by passing the tape through an elongated, heated drying chamber. The resulting product is a ceramic tape that is aptly described as "leather hard." This tape can be punched or sliced into the shape and size desired for the substrate or other item and then be fired at high temperature (e.g. 1500.degree. C.) to produce a rigid ceramic article.
The high vapor pressure solvents used in the conventional process for producing ceramic articles are generally toxic, may be flammable and have low flash points, thus creating a health hazard and environmental problems and necessitating reclamation of the solvent evaporated from the slip during drying. In addition, when the solvent is evaporated by heating, shrinkage and binder migration occur making it difficult to control the thickness of the product and causing stresses to develop in the green ceramic product. The stresses cause deformation of e.g., a tape into a curved surface with a saddle shape cross-section and are frequently sufficiently severe to cause cracking.
The critical factor limiting the production speed of dried, "green" ceramic tape (i.e. leather hard tape) is the rate of evaporation of solvents from the slip. Long drying times are necessary to remove the organic solvents due to skin drying and the high liquid to solid ratio. As a result, control of the thickness profile during casting is difficult, and only limited thicknesses can be attained. In addition, the slip is highly sensitive to air flow and heat distribution while drying, which leads to flow lines, surface defects and thickness variations.
It is a primary object of the present invention to provide a method of making ceramic items which permits a more rapid rate of production and reduces the stresses and deformations in the "green" ceramic material which occur due to the evaporation of solvents from the slip during curing.
A further object of the present invention is to provide a method of making ceramic items which eliminates the need for high vapor pressure organic solvents and the safety and environmental problems associated with their evaporation and reclamation.
A still further object of the present invention is to provide a method of making ceramic items which provides improved control over the thickness of the ceramic items.
Another object of the present invention is to provide a method of making ceramic items in which the articles can be formed by a variety of methods, including casting, extrusion, and molding.
Yet another object of the present invention is to provide a method of making ceramic items in which both sides of the item may be protected by a carrier surface during curing.